Add functions to the abstract base class

The plan is:
1. On "fit" or "seq fit", emit a signal functionUpdateRequested.
The signal has a bool to say if it's sequential or not.
2. MuonAnalysisFitFunctionHelper sinks the signal and gets the function
string from FunctionBrowser.
3. MuonAnalysisFitFunctionHelper passes function string to
MuonFitPropertyBrowser and calls runFit() or runSequentialFit().

The overall aim is to sync the function with the user's input before a
fit is run.

re #15518